Workflow to Perform Mapping Surveys using drones: WingQuad3
Nowadays, drone technology is being used for various purposes related to the collection of georeferenced information on a small or large scale such as topography. Photogrammetry now allows us to apply it's results to base and thematic cartography through the measurement of land and objects through aerial images acquired, in this case, by aerial robots or drones.
One of the most relevant projects that has been carried out was a survey performed in November 2019 in Guayaquil city (Ecuador) by a company responsible for conducting topography studies and the provision of potable water to the city. The main objective was to collect information in the field with our Wingquad3 of a certain area of the city to determine the number of properties that will benefit from the potable water and sewerage service.
It is important to emphasize that for the collection of reliable and precise results, an appropriate methodology must be applied. Thus, taking the aforementioned case study, we used the methodology shown below:
Phase 1: Recognition of the study area
When carrying out mapping surveys, it is very important to analyze the physical and weather conditions presented by the site like relief, precipitation, accessibility, etc., which will later help us for a correct work planning. This procedure is done either on-site or remotely, with tools such as google earth or satellite images if available.
For the area in question, it was determined that it is a dry, relatively flat area with slopes not greater than 15%, and with total accessibility to the entire sector where the survey was planned.
Phase 2: Field Work Planning
Once the study area is analyzed, work planning begins. For our case study, 4 engineers participated to carry out the work faster : 2 Civil Engineers and 2 Geographical Engineers.
A group of 2 was responsible for the placement of ground control points, located in strategic sites and homogeneously distributed to adjust the generated models and obtain precise results. The remaining team members were responsible for flight planning, information gathering, data processing and results generation such as point cloud, orthomosaic, digital surface model, digital terrain model, contour lines, and digitization and thematic cartography.
A double frequency GNSS equipment was used with which homogeneously distributed ground control points (GCPs) were placed in strategic sites to adjust the results and data collected. Also, our Wingquad 3 was used, which can collect information of an area of up to 350 hectares in one single flight, with an autonomy of 60 minutes. The drone incorporated a 24-megapixel RGB sensor that matched the resolution and scale that was needed.
Phase 3: Field Work
Based on the assessment of the study area, it was estimated that it was necessary to place 7 ground control points to reach the accuracy and resolution required, and the approximate places where they were placed were determined.
On the other hand, flight planning was carried out in our LAS-AirRails Ground Control Station (GCS), where the flight altitude, the sensor to be used and the strategic point from where we made the flight were determined.
First, 2 members of the team went to the study area to take the GPS coordinates in the field, which were marked and painted with clearly identifiable marks visible from the air. This marks will serve as points of photogrammetric support.
On the other hand, two drone flights were planned at 220 meters of altitude, with a 24 MPX RGB sensor, acquiring a GSD (Ground Sample Distance) of 5.32 cm/pixel.
Phase 4: Data processing and Results generation (at the office)
Once the fieldwork was completed in 1.5 days, the information was processed, both the data obtained with the GNSS equipment and the images obtained with the drone. In the first case, the coordinates obtained with a control point processing software were processed based on 2 points of the GNSS Continuous Monitoring Network of Ecuador –REGME.
Simultaneously, image processing was performed with the Pix4D Mapper photogrammetric processing software with which we obtained the following results:
Point cloud in LAS format.
It was obtained from 65,613,272 3D densified points
Average (points per m3): 22.07
Orthomosaic in GEOTiff format with a resolution of 5.15 cm/pixel.
Digital surface model in GEOTiff format with a resolution of 5.15 cm/pixel.
Digital terrain model in GEOTiff format with a resolution of 25.75 cm/pixel.
Contour Lines in shp format with 1 meter interval
Subsequently, the results were refined using geographic information systems necessary for this purpose. Finally, with the generated orthomosaic, the digitization of the houses and blocks located on the site was performed.
All the results delivered to the contracting company will serve in the future to estimate the possible number of beneficiary properties that will receive potable water and sewerage.
By: Gabriela López
Geographical Engineer & Environmental Management
LAS Technical Coordinator & Sales Engineer
For more information visit: www.latitudeas.com
Phone: +1 347 960 6444